The present disclosure relates to an electric motor-driven truck.
JP 2009-303283A discloses a technique of measuring a phase current flowing in a three-phase alternative current motor and calculating a voltage command by feedback control. An inverter is used to drive the electric motor. The inverter switches on and off a plurality of switching elements based on a pulse width modulation control signal from a PWM converter and applies a DC power in the form of a three-phase AC power to the electric motor. Each of the plurality of switching elements allows the current to flow and shuts off the current in synchronism with rotation of the electric motor.
In general, allowing the current to flow in the electric motor rotates the electric motor and accordingly does not cause the current to continuously flow in any of the plurality of switching elements. In the state of torque insufficiency, however, allowing the current to flow in the electric motor fails to rotate the electric motor. In this case, the current continuously flows in one of the plurality of switching elements. The continuous flow of current raises the temperature of the switching element. The switching element has a failure at the excessively high temperature.
In general to prevent such a failure, the temperature of the switching element is measured and the current value flowing in the electric motor is forcibly set to zero when the measurement value exceeds a threshold value. Forcibly setting the current value to zero the torque becomes zero. Accordingly, it is preferable to avoid such a forcible control whenever possible.
In order to avoid such a forcible control, a possible procedure measures the temperature of the switching element and performs feedback control to prevent the measurement value from exceeding the threshold value. Such feedback control, however, has a limitation in response speed. Additionally, the switching element is a small component and has a small heat capacity. Accordingly, the temperature of the switching element is likely to exceed the threshold value, prior to activation of the feedback control.
In order to ensure normal activation of the above feedback control, it is preferable to estimate the torque level that is likely to cause a torque insufficiency. For example, in the case of an electric motor mounted on the vehicle for driving, the torque insufficiency may occur when the vehicle rides over a large bump or when the vehicle starts on an uphill. These phenomena are affected by the vehicle weight. In the case of, for example, a passenger vehicle having a small variation in vehicle weight, the value of vehicle weight is regarded as a fixed value and is used for feedback control to prevent the temperature of the switching element from exceeding the threshold value.
A truck, on the other hand, has a significantly larger variation in vehicle weight than that of the passenger vehicle. Accordingly, it is impractical to regard the vehicle weight as a fixed value. The above feedback control may be implemented by measuring or estimating the vehicle weight. Such measurement or estimation, however, needs time and labor or needs complicated calculations and is thus unpreferable.
By taking into account the foregoing, an object is to avoid overheating of a switching element in an electric motor-driven truck by a simple technique.